The invention relates to a method for producing a rim for a pneumatic tire for vehicles.
From German Patent Document DE 32 39 675 C2, a method is known for producing a one-piece vehicle wheel of a light metal which consists of a hub part and a pneumatic-tire rim and, in the case of which a wheel disk is produced from a round blank by means of a compression operation and, for forming a rim base, the remaining ring edge is split and, by means of a subsequent forming operation, the final rim base shape is produced with legs of different widths while the thickness is approximately the same. It was found in practice, that, when producing such pneumatic-tire rims, unbalance problems occur as a result of the xe2x80x9csplittingxe2x80x9d, which make the produced pneumatic-tire rim useless.
It is customary nowadays to produce the vehicle wheel from two components, specifically, on the one hand, a rim disk, which can be cast or rolled, and the actual rim well which is produced by a compression operation. The two components are then welded together. Because the rim disk must additionally be adapted to the purchasers"" aesthetic demands, but the rim base does not have to take this demand into account, such a process is more cost-effective than producing the actual vehicle wheel in one piece. The rim base can always have the same construction, but the rim disk can be produced in smaller series corresponding to the purchasers"" wishes.
When producing the rim base, which is subsequently welded to the rim disk, the current practical approach is such that a round blank is used which has an outside diameter sufficient for producing the rim base after the compression operation was carried out which is applied to the round blank on the outside. In which case therefore, the center hole of such a round blank is extremely large. Since the round blank is punched out of a sheet metal plate, this results in high expenditures with respect to material as a result of the subsequent punching-out of the center hole. The area of the punched-out center hole is discarded, which results in an expensive and high-expenditure process.
It is an object of the invention to suggest a production method for a rim of a pneumatic tire in which the rim base is produced in a compression operation, but the rim disk is produced as a cast or rolled part, whereby the material expenditures for producing the rims can be considerably reduced in comparison to the methods known in practice.
This object on which the invention is based is achieved by the placing a blank in a compression mold having a cavity and using a pressure roller to press the blank against the at least partially contoured interior side of the compression mold.
The blank can be formed by a round blank, by a forged part, a cast part, a turned part, or the like.
A forged part can be preformed, for example, by cold forming on a so-called wobble press, whereby advantages are achieved in comparison to the other above-mentioned types of blanks.
Thus, the material used for such a forged part is considerably less in comparison to a round blank because, by the cutting-to-size alone, 30 to 40% of the material will occur as waste. In the case of a forged part produced on a wobble press, this fraction is considerably lower. By using a blank produced in this manner, the production costs are considerably reduced in that, for producing the blank, a block is used which is cut off a rod, for which light metal is particularly suitable.
Since the production of the preformed blank as well as the compression operation for producing the rim base represent a cold forming, advantages are also obtained thereby in comparison to, for example, a cast blank. Because of the resulting higher strength caused by cold work-hardening, the wall cross-sections can have smaller dimensions, which reduces the production costs and causes more favorable running characteristics of the pneumatic-tire rim.
In this context, it is provided according to an advantageous embodiment of the invention to calibrate the wall thickness of the rim base immediately during the pressing of the blank onto the interior side of the compression mold or subsequently corresponding to the desired rim requirements; that is, to construct and/or contour the walls in different thicknesses.
The invention also makes it possible to produce the rim base in one piece, so that a welding-together, as required in the case of a rim base which, for cost reasons, has a two part construction, can be eliminated. Ideally, such a one-piece rim base is particularly suitable for tubeless tires because of its airtightness.
While, for producing the known light-metal rims, on which the highest demands are made with respect to their quality, approximately 60 operating cycles are required, as a result of the invention, a light-metal rim can be produced at a fraction of these labor expenditures.
It is a characteristic of the method of the invention that a blank is used as the basis whose outside diameter corresponds approximately to the outside diameter of the rim, so that the previously occurring losses of material are avoided.
According to an advantageous further development of the invention, a round blank, which is clamped between a main spindle and a pressure pad of a system and which is used as a blank, is placed by means of a compression process against the compression mold constructed as a cavity; that is, is placed from the inside into the cavity. When the round blank is placed against the interior side of the cavity, not only the round blank is adapted to shape of the cavity, but simultaneous, the wall thickness of the round blank can be calibrated, as mentioned above, corresponding to the requirements of the rims. Thus, the wall thickness of the formed rim base, originating from a round blank which has a uniform material thickness, viewed along its length, can have different wall thicknesses where a relatively large wall thickness is required in the area of the rear rim flange, and a relatively large wall thickness has to be provided in the area of the marginal edge of the rim base facing away from the rear rim flange, the wall thickness between these two areas can be smaller.
Subsequently, the thus produced rim base is welded to the actual rim disk. This type of process has the disadvantage that certain unbalance phenomena may occur during the welding which require a reworking.
In order to avoid this disadvantage, it is suggested according to a further development of the method that the round blank, which is fixed between the main spindle and the pressure pad, is placed from the inside on a compression lining constructed as a cavity, so that a first drawn element is established. In this case, the first drawn element can have an identical wall thickness viewed along its entire length, which wall thickness corresponds to the wall thickness of the initial round blank.
In a second operating cycle, this first drawn element is then welded to the rim disk, and this unit consisting of the rim disk and the fixed first drawn element is now clamped in between a main spindle and a compression lining. By means of a compression method, the first drawn element is formed to produce the actual rim base. The other compression lining has the shape of the rim base, so that the shape of the rim base is produced by the placing of the first drawn element and simultaneously, as a result of stretching, the first drawn element wall thickness is calibrated to correspond to the requirements of the actual rim base during the production.
It was found that, as a result of this method of operation, the possible unbalance phenomena occurring because of the welding of the first drawn element to the rim disk are compensated and eliminated by the actual subsequent compression process. Although as a result of this process, a multi-stage operating method is provided, it also leads to a wheel rim which can be used immediately.
Here also, the outer circumference of the initial round blank, before the production of the first drawn element, has an outside diameter which corresponds approximately to the outside diameter of the actual rim base. Thus, the rim base can have a larger length than the outside diameter of the actual round blank because, as a result of thinning of the material, a stretching of a first draw takes place.
The high expenditures of material, which are still customary in the prior art, are also avoided in the case of this method, in which case it can generally be said that a saving of material of approximately 40 to 50% can be achieved in comparison to the methods so far used in practice.